The present invention relates to a method for the production of an extruded, polystyrene-based resin foam plate for use as a heat insulator for walls, floors, roofs and so on of buildings or as a tatami mat core, and to an extruded polystyrene-based resin foam plate obtained by the method.
Because polystyrene-based resin foams have excellent heat insulating property and desirable mechanical strengths, plates thereof formed to have a specific width have been widely used as heat insulators. One known method for production of such a foam plate comprises the steps of heating and kneading a polystyrene-based resin material together with a nucleating agent, mixing the kneaded mixture with a physical blowing agent, and extruding the mixture from a high pressure zone into a lower pressure zone.
As the blowing agent for use in the production of the foam plate, a chlorofluorohydrocarbon (which will be hereinafter referred to as “CFC”) such as dichlorodifluoromethane have been hitherto widely used. However, in place of CFCs having a possibility of destroying the ozone layer, a hydrogen atom-containing chlorofluorohydrocarbon (which will be hereinafter referred to as “HCFC”), which has a smaller ozone destroy coefficient, is increasingly used in recent years.
However, HCFCs, whose ozone destroy coefficient is not 0, are not without possibility of destroying the ozone layer. Thus, it has been studied to use a fluorohydrocarbon (which will be hereinafter referred to as “HFC”) having an ozone destroy coefficient of 0 and containing no chlorine atom in the molecules thereof as the blowing agent.
However, HFCs have a large global warming coefficient and thus still have a room to be improved in view of the preservation of the global environment.
Thus, it is desired to produce a polystyrene-based resin foam plate using a blowing agent having an ozone destroy coefficient of 0 and a small global warming coefficient.
Isobutane, which has an ozone destroy coefficient of 0 and a small global warming coefficient, is an excellent blowing agent. Also, since isobutane has a permeation rate to polystyrene which is much lower than that of air, a foamed heat insulation plate produced using isobutane can maintain the heat insulating property at the time of production over a long period of time. However, since isobutane in a gas phase has a thermal conductivity which is lower than that of air but higher than that of CFCs, HCFCs or HFCs which have been heretofore used, it is difficult to obtain a heat insulating property comparable to that given by HFCs and so on by using isobutane as a blowing agent. Also, since isobutane has a high flammability itself, it is considerably difficult to impart flame retardancy to the resulting foam.
In recent years, an attempt has been made to use an ether type blowing agent such as dimethyl ether together with pentane. The purpose of using such a mixed blowing agent is to obtain a polystyrene-based resin foam plate which has an ozone destroy coefficient of 0 and a small global warming coefficient and which is excellent in flame retardancy, heat insulating property and light in weight. To achieve this purpose, there is proposed a foam, which is produced using a large amount of a flammable blowing agent such as pentane and which has such a special cell structure for improving the heat insulating property that cells of large and small sizes are mingled. Additionally, the foam contains a specially formulated flame retardant for improving the flame retardancy. However, this method has problems in productivity and stable flame retardancy.